In Blender, the best way is to delete the materials, but keep the UV mapping. Press F9, you should see "UV Texture ... New" with one named "UVTex" below (you can change this default name and use multiple maps but then you have to make sure the right one is active). That is what matters for image based .wrl and Shapeways.

To clean up the render materials: delete them in F9 where it says 1 Mat 1, it should say 0 Mat 0, so press the button Delete that's below. And F5 buttons should show no materials linked, if you did it right. Then exporter will work without any manual tweaks about materials.

With your .blend I think it works for one object because it puts a material definition in one object and a reference to it (instead of the full material def again) plus the image in the other object, and Shapeways ignores that reference.

Youknowwho4eva suggests to fix it by hand, but if you do it right in Blender it should show:

I tried exporting it to verify, but it failed. Only one letter has image (the other has empty appearance block)... which leads us to another thing you've to do: select the R, enter edit mode and select vertices, select the image in image window so the vertices are associated to it.

So summing up: no render materials linked (in F9, 0 Mat 0), right UV texture active (F9 again, one UV Texture at least, it'll be created when you unwrap if needed), all faces associated to images (image area shows right one while you select things in edit mode).